Seeking Quieter Efficiency for Electric Vehicles

One of the pleasures of driving an electric car is its quiet operation. But the peace that motorists gain comes at a price for automotive designers. Without the constant rumble of an internal combustion engine masking other noises, they have to find a way to silence normal vehicle sounds that had previously gone unnoticed. Unfortunately, the fixes for those unwanted sounds often have a negative impact on the efficiency of electrical vehicles (EVs).

Some researchers in the United Kingdom believe that composite materials may provide a solution. The National Composites Centre (NCC), which partners with industry to drive innovation in composite design and manufacture, is working with the engineering consulting firm Drive System Design on a 12-month project to study the use of composites for integrated electric drive units (EDUs) in electric vehicles. The goal is to increase the efficiency of the EDUs without negatively affecting their noise, vibration and harshness (NVH) characteristics.

The electric drive units in EVs consist of a motor, transmission and inverter, which converts the DC power in the battery to the AC power required by the electric motor. In fossil-fuel vehicles, these three components may be located in different areas of the engine, but in EVs, where the emphasis is on weight reduction, the three subsystems are more often being integrated into one compact housing, which is the EDU.

“The advantage is really around package density and how much performance you can have within a given space or mass,” explains Markus Hose, head of mechanical engineering at Drive System Design (DSD). The problem is that in the initial design stages of EDUs, the focus is almost exclusively on their efficiency. Consideration for total system NVH often occurs late in the development program, and the solution often reduces the vehicle’s performance. “If you have a noisy motor, for example, you typically have to do things to it that will reduce its efficiency,” Hose adds.

During the year-long research project, the partners will investigate whether the use of composite materials can help balance NVH and EDU efficiency. “We wanted to start with the NVH characteristics at the beginning of the design stage rather than address them at the end,” Hose says. If rotor covers made from composites materials dampen NVH up front, designers will have more freedom to increase the power in, and thus the efficiency of, a compact EDU.

In the initial research phases, NCC researchers have been studying the properties of several different kinds of composite materials. “The first question we’ve got to ask ourselves is which composites give us which performance benefits in which NVH characteristics,” says Hose. One challenge is to understand the damping characteristics of different types of composites. (Damping is the ability of a material to reduce the resonances that cause noise and vibration.)

To be effective in an EDU, the composite material must have good damping characteristics while retaining the desired strength and stiffness properties. “There is typically a bit of a tradeoff between stiffness and damping. Things that might be really well damped, like a piece of paper, won’t be very good handling loads in the gearbox or motor,” Hose says. To find the right materials, NCC has been investigating long-fiber thermoplastics with various combinations of polymers and fibers, including everything from natural fibers like hemp to high-performing carbon and aramid fibers.

Working with composites can be challenging simply because of the many possibilities they offer. “There is so much information out there, and there are so many permutations,” Hose says.

DSD is assisting NCC with its expertise in transmission and motor design and with the design parameters for the composite materials. The list of requirements has been set deliberately high in the hopes that it will stimulate innovation. “That’s when those eureka moments can happen,” Hose adds.

During the project, DSD will also be using its modeling capabilities to virtually test how changing some structural elements in an EDU – switching out an aluminum mount to a composite mount or a steel ball bearing case to a composite case, for example – will impact the NVH characteristics in a vehicle.

In addition, NCC and DSD are looking at the feasibility of using composite banding to improve motor design in production vehicles. Increasing the strength of the magnets in a rotor improves a motor’s power and efficiency, but also adds mechanical strain to the rotor casing. If manufacturers could find a cost-effective way to wrap that casing in a composite band, providing additional strength, they could increase the motor performance for all the EVs they produce.

DSD and NCC anticipate wrapping up their composites research sometime in the fall. While the results will answer some questions about composite materials’ ability to dampen sound, they are likely to introduce even more. The next step could be finding a company interested in doing real-world testing of the composite rotor components. “We will have moved the game in our thought leadership a lot,” says Hose. “I think we will know better how we can make some big leaps in designing more interesting combinations of electric drive units.”

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